Two things. My copy of I6 Ravenloft. I still LOVE that module. Wow! I have played and
DM'ed that thing and it's just too good. It's not even in fine condition, but I don't care.
The other was, and I'm not sure if this really counts, the certificate of victory for coming in first place in a AD&D tournament. The victory was sweet, but the fun our team had working together to make it happen was an adventure I'll never forget.

Truth is worth finding and life is too short to work for money.